This invention relates to the field of lightwave communication systems and, more particularly, to overcoming optical impairments in short haul lightwave communication systems employing optical amplifiers.
To meet the increasing demands for more bandwidth and higher data rates, wavelength division multiplexing (WDM) is being used extensively in long haul optical transmission systems and is being contemplated for use in ,short haul applications, such as metropolitan area networks and the like. As is well known, WDM combines many optical channels of different wavelengths for simultaneous transmission as a composite optical signal in an optical fiber.
Optical amplifiers are commonly used in lightwave communication systems as in-line amplifiers for boosting signal levels to compensate for losses in a transmission path, as power amplifiers for increasing transmitter power, and as pre-amplifiers for boosting signal levels before receivers. In WDM systems, optical amplifiers are particularly useful because of their ability to amplify many optical channels simultaneously. Although rare earth-doped fiber optical amplifiers, e.g., erbium-doped fiber amplifiers, are commonly used in WDM systems, semiconductor optical amplifiers are being contemplated for use in WDM short haul applications like metropolitan area networks and so on. In particular, semiconductor optical amplifiers appear to be a viable alternative to the more costly erbium-doped fiber amplifiers. A typical short- haul WDM network includes a plurality of network elements interconnected by optical fiber. It is common to include optical amplifiers in the network elements in order to boost the lightwave signal power of signals traversing the optical fiber between network elements. However, the amount of amplification must be properly controlled because of optical impairments that are dependent upon signal power. In addition to affecting the transmission of optical signals, these optical impairments can also adversely affect the operation of optical components in the transmission path.
For example, Rayleigh backscattering is a well-known problem in which unwanted reflections are produced as an optical signal propagates through an optical fiber. In Rayleigh backscattering, the power level of the backscattered signals can be especially detrimental to the operation of optical amplifiers, such as semiconductor optical amplifiers, causing instabilities in operation, adding noise, and so on.
Non-linear effects can also cause problems in optical transmission. For example, Stimulated Brillioun Scattering (SBS) is a known phenomena which occurs when the power level of optical signals exceeds a certain threshold referred to as the SBS threshold. Briefly, SBS is a stimulated scattering process which converts a forward travelling optical signal into a backward travelling component of the optical signal which is also shifted in frequency. Among other problems, SBS results in increased backward coupling into optical components in the optical fiber path, which can affect operation of the components. For example, a backward travelling component can cause instabilities in optical amplifier operation. Other fiber non-linearities, e.g., four wave mixing, cross-phase modulation, self-phase modulation, Raman effect, and so on, are also well-known and can also be problematic in optical signal transmission. The network environment and topology are significant factors in determining when and to what extent the aforementioned problems will arise. Accordingly, proper design of a system is required in order to operate in the presence of such conditions. For example, long haul optical line systems typically have fiber spans of 80-120 kilometers between optical amplifiers without any intervening network elements. In these systems, optical isolators are typically employed to block unwanted back reflections of optical signals that would otherwise enter back through the output of the optical amplifiers. Moreover, the length of fiber spans serves to attenuate both forward propagating signals as well as backward travelling components, thereby reducing the occurrences of the aforementioned optical impairments.
By contrast, short haul optical systems, such as metropolitan area networks, have much shorter fiber spans. These short haul networks therefore cannot rely solely on the length of fiber spans to provide the necessary attenuation. Furthermore, these short haul networks typically have a higher density of network elements in a more geographically confined area, e.g., more closely-spaced optical amplifiers and network elements. Placing optical isolators at each network element location is very costly and highly undesirable in the cost-sensitive, short haul environment.
The challenges associated with operating optically amplified short haul networks in the presence of the aforementioned optical impairments are further complicated by the dynamic nature of short haul networks. For example, short haul WDM networks generally include a plurality of network elements capable of adding/dropping, routing, and cross-connecting optical signals. Losses introduced by these network elements can be significant. However, boosting gain of optical amplifiers to compensate for these losses can cause the system to be even more susceptible to the aforementioned optical impairments, e.g., exceeding SBS thresholds, causing higher intensity back reflections, and so on.
The effects of optical impairments on optical signal transmission in a lightwave transmission system are substantially reduced according to the principles of the invention by positioning optical amplifiers and network elements in respective upstream-downstream combinations. More specifically, by placing an optical amplifier at a position upstream from its corresponding network element, sufficient amplification can be provided by the optical amplifier to compensate for losses introduced by its corresponding network element. Advantageously, the corresponding downstream network element provides sufficient attenuation of the forward travelling lightwave signals so that power-dependent nonlinear effects in the optical fiber do not significantly distort the lightwave signals. Moreover, because of the downstream location of the network element in relation to its corresponding network element, the network element substantially suppresses backward travelling optical signal components such as those caused by Rayleigh backscattering, Stimulated Brillioun Scattering (SBS), and the like. As such, the network element prevents unwanted back reflections and back scattered signals from affecting operation of its corresponding optical amplifier.
Costs associated with installing and operating lightwave transmission systems are also substantially reduced according to the principles of the invention. In particular, costly optical isolator components are no longer needed at the output of every optical amplifier because the respective downstream network elements effectively perform an isolation function. Less expensive semiconductor optical amplifiers can also be used to further reduce system cost. These cost savings can be especially advantageous in the cost-sensitive short haul WDM network environment.